Subtract.
There are many ways to solve this problem. Let's see two ways we can solve it. Place Value We can think in terms of hundredths: $\begin{aligned} &\phantom{=}84- 2.72\\\\ &=84.00-2.72\\\\ &=8{,}400\text{ hundredths} -272\text{ hundredths}\\\\ &=8{,}128\text{ hundredths}\\\\ &=81.28 \end{aligned}$ Place value table Let's line up the numbers by their decimal places. Tens Ones $.$ Tenths Hundredths ${8}$ ${4}$ $.$ ${0}$ $0$ $2$ $.$ ${7}$ $2$ Now, let's subtract each place value column. Tens Ones $.$ Tenths Hundredths ${8}$ $\overset{3}{\cancel{4}}$ $.$ $\overset{9}{\cancel{0}}$ $\overset{10}{\cancel{0}}$ $-$ $2$ $.$ ${7}$ $2$ $\underline{~~~~}$ $\underline{~~~~}$ $\underline{~~~~}$ $\underline{~~~~}$ $8$ $1$ $.$ $2$ $8$ $81.28 = 84 - 2.72$